Two former Parramatta Eels players are accused of harbouring semi-automatic weapons and possessing more than half-a-million dollars in cash after dramatic arrests in Sydney's Centennial Park yesterday.

UK police arrest four men suspected of plotting terror attacks

Counter-terrorism police have arrested four men about 50km north of London on suspicion of plotting terrorism acts.

The men were arrested in Luton before 9am Wednesday (8pm AEDT) on "suspicion of committing offences under the Terrorism Act", UK Metropolitan Police said in a statement.

Police believe the men have been involved in "the commission, preparation and instigation of acts of terrorism".

All of the men are aged in their thirties and have been taken into custody at an unnamed London police station.

The arrests were made as part of a joint counter-terrorism operation involving the police Counter Terrorism Command SO15, and the Eastern Counter Terrorism and Intelligence Unit ECTIU, Metropolitan Police said.

Police are also searching seven Luton properties and some vehicles as part of the investigation.

The investigation is proactive and is not connected to the recent attacks in Paris, Metropolitan Police said.